A standard central motor-vehicle lock system has a plurality of door latches on the individual doors, hatches, trunk lids, and the like that are each operable by a respective power actuator and also by a manual mechanism. The power actuator can include a hydraulic, pneumatic, or electric motor, and the manual mechanism is almost always a lever linkage.
In a standard system such as described in my U.S. Pat. No. 4,342,209 each door latch has a locking level displaceable between a pair of end positions corresponding to locked and unlocked conditions of the respective door. This lever is connected on the one side via a rod to the inside unlocking button in the case of a door and on the other side via another such rod to the power actuator which is mounted in the door at some remove from the latch. Thus either the knob or the actuator can be operated to lock or unlock the door.
It has become common to provide a standard central locking system with a so-called antitheft feature. When set in the antitheft mode it is impossible to unlock the vehicle doors even by manual actuation of the inside knobs. Thus a person who breaks a window or otherwise gains forcible entry to the vehicle cannot open its doors.
Such a system is typically incorporated into the above-described power actuator by either driving the element that acts via the rod on the unlocking lever with such a large mechanical advantage that forcible reverse-driving of this motor is impossible, or by providing a system which positively freezes this actuating element. Either way action on the inside knob will be countered by the actuator via the rod connecting it to the latch locking lever.
A weakness of such a system is that an extreme force exerted on the inside knob, for instance by prying on it, can bend or break the rod connecting the locking lever to the actuator. This problem is particularly great when the knob must be pulled away from the latch to unlock the door, as the amount of force such a rod can withstand in tension is considerably greater than the forces that the rest of the elements can resist. The only way to make the system secure against such attack is to dimension all the parts the entire connection between the actuator and the locking lever so they are very strong. This augments the weight and cost of these elements considerably.